UDP transport with broadcast support, -udp and -iface flags on info

cmd/picomap/main.go

@@ -73,12 +73,16 @@ func printInfo(dev string, info *client.ResponseInfo) {
 func cmdInfo(args []string) error {
 	fs := flag.NewFlagSet("info", flag.ExitOnError)
 	udpAddr := fs.String("udp", "", "connect via UDP to this IP address")
+	iface := fs.String("iface", "", "bind to this network interface (for broadcast)")
 	fs.Parse(args)
 
 	if *udpAddr != "" {
 		log := slog.With("addr", *udpAddr)
+		if *iface != "" {
+			log = log.With("iface", *iface)
+		}
 		log.Info("connecting via UDP")
-		c, err := client.NewUDP(*udpAddr, 2*time.Second)
+		c, err := client.NewUDP(*udpAddr, *iface, 2*time.Second)
 		if err != nil {
 			return err
 		}

lib/client/udp.go

@@ -5,30 +5,91 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"syscall"
 	"time"
+
+	"golang.org/x/sys/unix"
 )
 
 const PicomapPort = 28781
 
 type udpTransport struct {
 	conn *net.UDPConn
+	addr *net.UDPAddr
 	buf  bytes.Buffer
 }
 
-func NewUDP(addr string, timeout time.Duration) (*Client, error) {
+func isBroadcast(ip net.IP) bool {
+	if ip.Equal(net.IPv4bcast) {
+		return true
+	}
+	ip4 := ip.To4()
+	return ip4 != nil && ip4[3] == 255
+}
+
+func interfaceIPv4(name string) (net.IP, error) {
+	ifi, err := net.InterfaceByName(name)
+	if err != nil {
+		return nil, fmt.Errorf("interface %s: %w", name, err)
+	}
+	addrs, err := ifi.Addrs()
+	if err != nil {
+		return nil, fmt.Errorf("interface %s addrs: %w", name, err)
+	}
+	for _, a := range addrs {
+		if ipnet, ok := a.(*net.IPNet); ok {
+			if ip4 := ipnet.IP.To4(); ip4 != nil {
+				return ip4, nil
+			}
+		}
+	}
+	return nil, fmt.Errorf("interface %s has no IPv4 address", name)
+}
+
+func enableBroadcast(conn *net.UDPConn) error {
+	raw, err := conn.SyscallConn()
+	if err != nil {
+		return fmt.Errorf("syscall conn: %w", err)
+	}
+	var serr error
+	raw.Control(func(fd uintptr) {
+		serr = syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, unix.SO_BROADCAST, 1)
+	})
+	return serr
+}
+
+func NewUDP(addr string, iface string, timeout time.Duration) (*Client, error) {
 	raddr, err := net.ResolveUDPAddr("udp4", fmt.Sprintf("%s:%d", addr, PicomapPort))
 	if err != nil {
 		return nil, fmt.Errorf("resolve %s: %w", addr, err)
 	}
-	conn, err := net.DialUDP("udp4", nil, raddr)
+
+	var laddr *net.UDPAddr
+	if iface != "" {
+		ip, err := interfaceIPv4(iface)
 		if err != nil {
-		return nil, fmt.Errorf("dial %s: %w", addr, err)
+			return nil, err
 		}
-	return &Client{transport: &udpTransport{conn: conn}, timeout: timeout}, nil
+		laddr = &net.UDPAddr{IP: ip, Port: 0}
+	}
+
+	conn, err := net.ListenUDP("udp4", laddr)
+	if err != nil {
+		return nil, fmt.Errorf("listen: %w", err)
+	}
+
+	if isBroadcast(raddr.IP) {
+		if err := enableBroadcast(conn); err != nil {
+			conn.Close()
+			return nil, fmt.Errorf("SO_BROADCAST: %w", err)
+		}
+	}
+
+	return &Client{transport: &udpTransport{conn: conn, addr: raddr}, timeout: timeout}, nil
 }
 
 func (t *udpTransport) Send(data []byte) error {
-	_, err := t.conn.Write(data)
+	_, err := t.conn.WriteToUDP(data, t.addr)
 	return err
 }